1. Field of the Invention
The invention relates to the beneficiation of sulfide ores of copper and nickel. More specifically, it relates to the selective flotation of copper and nickel minerals. A process has been discovered which permits the separate recovery of copper and nickel concentrates from such ores. Thus, the invention is useful where it is sought to process nickel- and copper-containing ore to recover separately a copper concentrate and a nickel concentrate.
The invention is particularly useful, for instance, in beneficiating ore that is found in the copper-nickel mineralization of the Duluth Complex orebody in northeastern Minnesota. This copper-nickel ore has resisted previous attempts to design a flotation process to recover a copper concentrate low in nickel.
2. Description of the Prior Art
A brief discussion of the Duluth Complex copper-nickel mineralization appears in Engineering & Mining Journal, Vol. 177, April 1976, at pages 80-83, together with a flowsheet for bulk treatment of the ore to achieve a rough separation of the copper and nickel minerals from gangue material. The discussion of the flowsheet does not suggest the selective flotation of the copper and nickel minerals using the sulfurous acid conditioning stage of the process in accordance with the present invention. Treatment of ore samples with the process from the disclosed flowsheet (with one rougher flotation stage and three cleaner flotation stages) produced bulk concentrates having copper contents of about 12%, corresponding to copper recoveries on the order of 85%, and nickel contents of about 2.5%, corresponding to nickel recoveries of about 68%.
The Duluth gabbro is also discussed in Transactions, Soc. of Mining Engineers, AIME, Vol. 241, December, 1968, at pages 421-431. This article discloses a flowsheet for bulk sulfide flotation followed by separation of copper minerals from nickel minerals, using traditional conditioning agents such as lime/cyanide and lime/British gum. In some runs the pH is optionally adjusted to about 6 with sulfuric acid. There is no disclosure or suggestion of conditioning the ore in aqueous sulfurous acid, as is evident from the results reported which show only mediocre recovery of copper in the final copper concentrate.
There are earlier processes in which mixtures of sulfidic copper and iron minerals are conditioned prior to flotation with sulfite derivatives, to promote the copper sulfides and depress the iron sulfides. Specifically, U.S. Pat. No. 1,397,703 discloses using a "non-alkaline electrolyte", such as a solution of sodium sulfite, and U.S. Pat. No. 1,678,259 discloses using an acid sulfite or sulfurous acid. Neither patent discloses whether nickel minerals were possibly included with the minerals under treatment, and the patents thus do not indicate or suggest the effect, if any, of the disclosed processes on nickel minerals. In addition, the statement on page 2, lines 41-44 of U.S. Pat. No. 1,678,259 that solutions of sulfur dioxide in water (i.e. sulfurous acid) are unstable and difficult to handle suggests to one skilled in this art not to include a sulfurous acid conditioning step in a continuous process for the separation of nickel sulfide minerals from mixtures with copper sulfide minerals.
Published South African Patent Application No. 71/1887, filed Mar. 23, 1971, relates to the froth flotation of copper sulfide minerals from ore that also contains carbonates, such as Ca(Mg)CO.sub.3, and magnetite, Fe.sub.3 O.sub.4. The application discloses that sulfurous acid added to the copper flotation circuit underflow, which is rich in the carbonate minerals and contains minor residual amounts of unfloated copper sulfide minerals, reacts with the carbonates to form bisulfite compounds such as Ca(Mg)(HSO.sub.3).sub.2 which activate the residual copper sulfides for recovery in a subsequent flotation stage. The sulfurous acid can also lead to precipitation of calcium and magnesium compounds. There is no disclosure of what, if any, nickel minerals are present in the ore under treatment or in the underflow which is treated with the sulfurous aicd, and thus there is no suggestion of the presently claimed discovery that conditioning a pulped copper-nickel ore in sulfurous acid can selectively activate the copper minerals and depress the nickel minerals. In addition, the Duluth gabbro orebody with which the present invention has been found to be effective does not contain the large amounts of carbonate minerals present in the ore discussed in the South African application; indeed, the Duluth gabbro contains no carbonates, or no more than trace amounts which do not affect the activation of the copper minerals by the sulfurous acid, and the South African application does not suggest that sulfurous acid is an effective conditioning medium for ores that are substantially free from carbonates.